Initial corrosion of AZ91D magnesium alloy in atmosphere containing SO₂

The initial surface morphology of corrosion layer, corrosion products and structure of rust layer for AZ91D magnesium alloy in SO₂ industrial polluted atmosphere were analyzed by metallographic observation, SEM and XRD. The results show that corrosion rate increases with increasing SO₂ content in the atmosphere. The initial attack has the characteristics of localized corrosion. Corrosion starts from a phase, and MgO and Mg(OH)₂ form at first, which provide a protective layer. SO₂ decreases pH in the electrolyte layer, accelerates dissolution process, and promotes the formation of MgSO₃·6H₂O and MgSO₄·6H₂O. These soluble corrosion products result in the loss of protection function and the acceleration of corrosion. Meanwhile, cracks appear with continuation of corrosion, which provide the paths for oxygen and other corrosion pollutants filtering into matrix, further resulting in severe localized corrosion.